The present invention is directed to a process for the hydrogenation of a thiophenic sulfur containing hydrocarbon feed, more in particular to dearomatization of solvents, middle distillates such as diesels, xe2x80x98white oilsxe2x80x99, gasoline and the like.
When hydrogenation catalysts are used in the hydrogenation of heavy feeds, such as petroleum distillates, often a problem presents itself in that the feed comprises sulfur and/or sulfur components, which adversely affect the lifetime of the catalyst. In such processes conventional hydrogenation catalysts are usually applied, for instance supported nickel catalysts. To reduce this problem of deactivation, much attention has been paid to the removal of sulfur-compounds from the gaseous or liquid feed prior to the hydrogenation.
In general sulfur impurities are present in feeds as mercaptans or thiophenes, more in particular thiophene, dithiophene, benzothiophene, dibenzothiophene, as well as substitution products thereof, which sulfur impurities can be hydrogenated to H2S using a sulfidized Coxe2x80x94Mo catalyst. The H2S formed is then removed from the feed by stripping, or by reaction with activated zinc oxide. This method is also known as hydrodesulfurization (HDS).
After separation of the hydrogen sulfide from the HDS-treated feed, and concentration of the hydrogen sulfide, it is usually processed to elemental sulfur in a conventional Claus process.
Under certain conditions, particularly when the sulfur content of the feed is not too high, it is economical not to remove the sulfur compounds completely prior to the hydrogenation process, but to allow a gradual deactivation of the catalyst, as the catalyst takes up the sulfur. After the deactivation of the catalyst has proceeded to a level that the activity becomes uneconomically low, the catalyst is replaced.
Since there is a need for processing feeds having a higher content of sulfur compounds than has been conventional up to now, which would lead to an unacceptably short lifetime of the catalyst, it is desirable to have a catalyst system with a reduced sensitivity to deactivation by sulfur compounds, i.e. having an increased on-stream time.
The product stream obtained from the HDS process still contains some sulfur. Typical sulfur levels of these product streams from HDS-units range from 0.1 to 300 ppm.
In the subsequent hydrogenation step, using a nickel catalyst, the major part of the sulfur is taken up by the nickel, as discussed above. Accordingly, the nickel catalyst will be deactivated in the course of time.
The on-stream time of a nickel catalyst in these systems depends i.a. on the amount of sulfur impurities or contaminants in the feed. However, it has been found, that the nature of the sulfur compounds also has a marked influence on the deactivation. Thiophenic sulfur has been found to have a much larger negative influence than mercaptans or hydrogen sulfide.
Thiophenic sulfur has been defined herein to include those organic compounds that include at least one thiophene ring, including, but not limited to thiophene, dithiophene, benzothiophene, dibenzothiophene, as well as substitution products thereof.
It has been proposed in EP-A 398,446 to provide a catalyst for hydrogenation and/or dehydrogenation having improved resistance against deactivation by sulfur and/or sulfur compounds, said catalyst comprising at least one hydrogenation component, at least one metal-oxide containing component and at least one support material, at least a part of the hydrogenation,component, as well as at least a part of the metal-oxide containing component being present on said support material as separate particles, and the particles of the hydrogenation component and the particles of the metal-oxide. containing component being homogeneously distributed in the catalyst.
Although this catalyst provided a distinct improvement over the prior art there still is a need for further improvement, especially in terms of the amount of sulfur that can be taken up by the nickel before it has to be replaced.
The present invention is based on the surprising discovery, that the thiophenic sulfur resistance of a nickel hydrogenation catalyst can be improved by contacting the entire thiophenic sulfur containing hydrocarbon feed with a platinum group metal (to be defined hereafter) prior to or simultaneously with contacting the said feed with the nickel catalyst.